US2668791A - Control of gas production in the vapor phase conversion of liquid hydrocarbons - Google Patents

Description

Fb- 9, 1954 w. w. HOLLAND 2,668,791

CONTROL OF GAS PRODUCTION IN THE VAPOR PHASE CONVERSION OF LIQUID HYDROCARBONS Filed Oct. 16. 1948 Nw( WN uw am. d l I W s H J. w u www w Sv y a m x@ MMV @s ssaaa Wann @uur wenn TIHIIZDNu mllz'am Wildland www Patented Feb. 9, 1954 CONTROL OF GAS PRODUCTION IN THE VAPOR PHASE CONVERSION OF LIQUID HYDROCARBONS William W. Holland, Baltimore, Md., assignor to The Gyro Process Company, Detroit, Mich., a corporation of Michigan Application October 16, 1948, Serial No. 54,946

9 Claims. (Cl. 1536-61) This invention relates to the conversion of heavier hydrocarbons into lighter hydrocarbons, and more particularly to the conversion of liquid hydrocarbons of higher boiling point and While held in the vapor phase into hydrocarbons of lower boiling range suitable for the production of improved motor fuels. It is directed specifically to the relatively high temperature conversion of hydrocarbons Without the production of an excessive amount of Xed gas which usually accompanies high temperature operations of this character.

When cracking hydrocarbon oils in the liquid phase, or the mixed liquid and vapor phase, the rate and degree of conversion increase with the rise in temperature, accompanied by a correspending increase in the production of coke and gas. The temperatures employed in liquid phase cracking are usually Within the range of 750 to 950 F., depending upon the character of the charging stock and the degree of conversion desired, the latter being the governing factor in the quality of the cracked product with respect to its anti-knock properties. It is customary, how ever, in such operations to employ only those temperatures which Will produce a moderately high octane distillate, without the production of large amounts of coke and gas, and thereby permit long operating periods on the part of the conversion apparatus and with 10W maintenance 5 costs. The anti-knock properties of the cracked products of such conversion are improved by the addition thereto of tetraethyl lead or by subsequent reiining steps such as alkylation or isomerization. By such methods the octane number of 1 the finished product may be increased to meet the requirements of motor iuel for commercial purposes.

When cracking hydrocarbons in the vapor phase, a distillate is obtained which possesses considerably higher anti-knock properties than liquid phase cracked distillate, so that it is not necessary to subject it to similar methods of after treatment in order to attain a given octane rat ing. This is due primarily to the inherent difierence in the chemical composition of the tWo types of distillate. No entirely satisfactory explanation has yet been advanced to account for this difference, but it is known that vapor phase cracked distillate contains certain hydrocarbons of cyclic and olenic coniigurations, not found in the liquid phase cracked product, which contribute materially to high anti-knock performance.

When cracking petroleum hydrocrabons in the vapor phase more fixed gas is formed than when cracking in the liquid phase because of the higher operating temperatures, which may range from 1l00 to 1350" F. when the production. of motor fuel is the primary consideration. Liquid fractions may be recovered from the gas, however, by methods of absorption, compression, refrigeration, polymerization and the like, which when added to the cracked motor fuel produce an overall yield comparable with that from liquid phase cracking. But instead of employing such methods to increase the yield of liquid products from vapor phase cracking, as liquid phase cracking processes employ methods of after treatment to improve the anti-knock properties of the distillate, it is more practical to control the gas production and thereby obtain a higher yield of liquid fractions, Without the necessity of additional steps for that purpose.

l have observed that as the hydrocarbon vapors pass through the conversion coil, the gas production increases with the progressive rise in temperaturc, While the production of liquid fractions within the motor fuel boiling range remains substantially constant, particularly in the last half or thercabouts of the coil, and that the gas production reaches its maximum point at the exit of the coil, coincident with the maximum temperature. I have noted also that if the rise in temperature is checked in the mid-section of the coil and maintained at a reasonably constant level thereafter, the gas production is markedly reduced, with a corresponding increase in the motor fuel production. It is concluded therefore that the excessive gas production commonly associated with vapor phase cracking is not due to nominally higher temperatures than those customarily employed in liquid phase cracking` considering the very much shorter time element, but to the excessively high temperatures reached at the end of the vapor phase cracking reaction.

It is, therefore, an object of this invention to provide a method and means to control the vapor phase cracking reaction so that the maximum yield of liquid products Within the motor fuel boiling range is obtained, and at the same time preserve the high anti-knock properties characteristic of the distillate.

Another object of this invention is to provide a method and means of temperature control in the conversion coil which will modify the progressive intensity of the decomposition reaction and thereby amounts of xed gas.

Another obiect of this invention is to provide a method and means of temperature control inhibit the production of excessive` a method and means to modify the intensity of the temperature conditions towhich thefconversion elements are exposed and thereby 1reduceequipment maintenance costs.

It is still another objectof this inventionto improve the economics of the V.p rocessgenerally by simplifying the several steps and by limiting the accessory equipment necessary to its complete performance, thereby aiordinga .widerdie erential between manufacturing costsfand the sales price of the nished products.

Other Objects of` this invention will become apparent vin thev further disclosures 'hereinafter Sl'1.f.0rl5h ,'Ijime and temperature are the important .factors in any cracking operation, particularly at the lhigh temperatures employed invapor phase.

crackingwand unless `they are controlledv within reasonable limits, the. reaction end vproducts .may varyconsiderably. Liquid phase cracking does not present a difcult problem with respect to .the reaction time becausefitjs gauged in terms of minutes, and soaking coil f sections or; drums are usually iprovided in order yto insure ample time for `completion of the reaction. Pressure is alsoV an important factor inliquid phase cracking, ,butY

chiey as a means .of reaching the desired tem"- perature without vaporizationo the oil under treatment. On the otherV hand, in vapor phase crackingthe time element is soshort, a matter of seconds only, that it isdiilicult todesign equipment which; under commercialoperating condie. tions can be controlled with suii'cient accuracy with respect'to the time-temperature relation.- ship to insure reasonably consistent results over extended operating periods. Anyj condition Which may'cause the hydrocarbon vapors to .be heated too long or too high. a temperature, even fora fraction of a second, is reiiected immediately in excess gas production.` The tendency seems always to have been in this direction in vapor phase cracking, due perhaps to the limited means provided for controlling the time-temperature reaction conditions. If tor any reasonthe rate of now `of the vapors ,through the conversion...

coil should vary orV if the furnace'temperature should rise or fall, the entire conversion operation vis thrown out o `balance, with consequent varying results in the end products.

Itis therefore a further object of this invention to provide means whereby the vapors passing through the conversion coil are,` maintained at asubstantially constant temperature independently of the rate of flow withinreasonable. limits or of moderate, nuotuations inthe furnace. temperature.

Means for Vcontrollingfurnace temperatures have-reached a degree of renernent and `dependability whichleave little to be desired. from that standpoint, but the temperature of the vapor within the conventionI coil hasY not been broughtv Y under. as completecontrol as could be desired..

Theoretically speaking,V as the hydrocarbon vapors pass through the conversion coil, the, temperature isA raisedvrapidly until yit'reaches the predetermined decomposition temperature of the vapors, at which point it is maintained for a predetermined time to complete the conversion reaction. In actual practice, however, this is not done in the present day conversion coil since the temperature of the vapor rises progressively during its passage throughout the length of the coil, leaving the=coil at itsfmaiimumitemperature. Here We are dealing with "the maximum `temperature reached in the coil, not with a properly predetermined decomposition temperature of the..vapor. .Itis lthis peak temperature that is mainlyresponsible for the high gas production in Vapor 'phase cracking processes. Quenching the vapors immediately upon emergence from the craclringrcoil-arrests'y the reaction so that no furitherfga-s isproduced, but it does not reduce the quantity of gas already formed in the coil. Quenching isa-means of terminating the reaction and in that respect only it affects the gas production.

Soaking coils have been used .asa `meansA Vof maintainingthe vapor at a constant temperature for the `required' time to eect the olesireddegreeV oiconversion but they have had'little or no effect upon the gas production because of thejdiicultyof controlling the temperaturein such-a bankof tubes when in the same furnace's'etting withYV the conversion coil, whether located either inthe radiant or convection sections.

of such a coil unless each tube were r'ed indey pendently. Similar resultsfhave been obtained with heavily insulated soakingdrums.

Ihave discovered that internal. cooling amore eiective means of'maintaininga constant temperature in a vapor phase conversion coil than external heating. In the latterrth'e temperaturevrises progressively from tube to tube throughout the length of the coil, reaching the maximum' temperatur-e at the exit-end. At no point in the coil is the temperature actually at a constant levelA requisite 'to the uniform cracking 0f the vapor, which condition results in varying:4 Y degrees of conversion and a -high gas production On the other hand, if the vapor is heatedto the predetermined temperature at which conversion is to be eiiected and is-checked and maintained Y contributes not only to a lower gas-production butV also to a less highly unsaturated distillate which isy more readilytreated with'the usual reliningagents, such as sulfuric acid.

I have found also that the rapidfrise in temperature inthe cracking coil may be readily checked at anydesired point by. injectingfinto the vapor stream a coolingagent such asa gaseous onlight liquid hydrocarbon (whichwmay` also enter into the cracking reaction) low pressuresteam, water or any other suitable cooling'- agent. Water is well adapted tothe purposebecause of its high .latent heat of. vaporization, the

small amountrequired to check the rise in tem perature and the ease with which itmay be introduced at any point or points in the conversioni coil. For example, when thevapor-hasreached the desired temperature. at any intermediate V l` point in the coil, the. temperature Will continueV to rise unless itis checked. At thisA pointa ne jet of water. isadmitted tothe coiLsuiicient-vin Even iff-located in a separate furnace, a constant temperature could not vbe maintained throughout the tlength amount only to arrest the temperature. Passing through subsequent sections of the coil, the temperature of the vapor again rises, at which point a second jet of water is introduced into the coil, and so on. With the third injection of water suicient time has elapsed, as a rule for the reaction to have been completed when the vapors leave the coil. It is thus possible to crack hydrocarbons in the vapor phase at a comparatively constant temperature, or within a given temperature range varying only a few degrees, whereas the temperature range during the cracking interval heretofore has been of the order of 100 to 150 F. variation.

The introduction of Water, steam or a hydrocarbon cooling agent into the conversion coil through a manifold system under thermostatic control presents no complications in construction or operating difiiculties. It is injected into the coil automatically only at the points where and when needed in order to maintain a substantially constant temperature therein. Not only does this means of controlling the temperature reduce the gas production and modify the unsaturated character of the distillate, particularly with respect to diolens, but less coke is formed under the milder operating conditions.

For a further understanding of the invention, reference is made to the accompanying drawings forming a part of this specification, wherein:

Fig. 1 represents diagrammatically a side elevational view, partly in vertical section, of a vapor phase cracking system employing internal cooling as a means of controlling the conversion coil temperature; and

Fig. 2 is a top plan view partly in horizontal section of the conversion coil and a header or manifold through which a cooling agent is delivered to the coil, together' with temperature oontrol devices.

Referring more particularly to the drawing, the numeral l represents a pipe line through which a hydrocarbon charging stock is delivered by pump 2 to a vapor phase conversion system wherein the hydrocarbon is decomposed for the primary purpose of producing a superior quality motor fuel, characterized by its high anti-knock properties, without the production of an excessive amount of fixed gas which usually accompanies high temperature operations of this character. The hydrocarbon charge is rst heated to its vaporization temperature in a continuous coil 3 suitably located in a furnace setting d, preferably in the convection section, and is discharged through line 3 into an evaporator 5 wherein the vapors are separated from any unvaporized heavy components of the charge, the latter being with drawn from the bottom of the evaporator through the valved line t and diverted from the system. The vapors leaving the top of the evaporator through line l' then pass through a bank of drying and preheating tubes t, also preferably located in the convection section of the furnace, wherein any liquid entrainment carried along mechanically by the vapors is converted into vapor, so that only dried, preheated vapors enter the conversion coil 9.

The temperature of the vaporous hydrocarbons at this stage of the operation reaches the point at which incipient cracking begins, and, according to current practice, it rises rapidly and progressively thereafter during the passage of the vapors throughout the length of the conversion coil. The rate of heat in-put is so rapid during the cracking interval that portions of the vapor,

6 particularly those adjacent to the inner walls of the tubes, become superheated and are decomposed far beyond the motor fuel stage, with a resultant excessive gas production. This condition is also largely responsible for the presence of certain highly unsaturated compounds in the cracked distillate which render its treatment by conventional methods and agents practically impossible.

To rectify this condition and to modify the decomposition reaction so that it will proceed smoothly and uniformly, means are provided to check the progressive rise in temperature by injecting a cooling agent into the vapor stream at one or more points in the course of its passage through the conversion coil so that it is maintained at a substantially constant temperature during the cracking interval.

Means suitably adapted to this purpose are shown in Fig. 2, wherein. the conversion coil 9 is tapped, preferably through the return bend fittings or junction boxes, to receive the pipes a, b and c which are connected with a header It, to which the cooling agent is supplied by pump li through line l2. Valves I3, lll and I5 provide regulatory means for admitting the cooling agent into the rapidly moving vapor stream in amounts only suiiicient to check the rise in temperature. By properly adjusting these valves, the temperature may definitely be controlled within narrow limits, thereby establishing substantially constant temperature conditions throughout the conversion coil.

Such temperature regulation may be accomplished by manual adjustment of the valves or by well known automatic temperature control devices such as those indicated at I6, il and i8. The temperature controllers are connected with suitable temperature-sensitive elements inserted into the respective sections of the conversion coil, and with the valves i3, ill and l5 in such a mam ner as to adjust the iow of the cooling agent passing through the valves in response to the variations in temperature of the vapors passing through the coil. A surprisingly small amount of the cooling agent is required to maintain a reasonably constant temperature throughout the conversion coil because of the high velocity of the vapor, the short time element between the cooling periods and the effectiveness of the cooln ing agent, especially when water is used for that purpose.

The cooling operation appears to have no ef feet whatsoever upon the anti-knock properties of the cracked distillate, although there is a very substantial increase in the liquid volume, with a corresponding decrease in the gas production. The distillate is of a less highly unsaturated character and responds more readily to conventional methods of treatment for the removal of undesirable constituents. A noteworthy feature of the process is the small amount of finely divided coke produced under such temperature controlled conditions.

Leaving the conversion coil through line 5', the products of the decomposition reaction enter a second cooling stage in a temperature arrester I8 where they are brought into intimate and direct contact with a cooling agent produced within the system, reducing the temperature to about 525 F. The mixture of gaseous and liquid products, together with some finely divided coke, then pass through line iii and into a separator 2d where the light fractions are removed overhead, preferably with the aid of steam, the tarry` ma- 7 .terials vand suspended .coke being Withdrawn from the bottom through .the vali/.ed line .2l and diverted fromthe system.

The overhead 4products leaving the .separator .through line .22 pass into ,a fractionating column .22, preferably .of the bubble tower type, Where .separation is eiected between the light and heavy 'fractions the latter being Withdrawn from the bottom of the column through line 2t, controlled Aby Val-Ve 2S, .and .sent to :fuel oil storage or used Afor ,other purposes. It may .also be returned to the .feed stock for reprocessing, if desired. fr liquid fraction is Withdrawn from an intermediate plate in the .bubble tower through line 2d and .cooler 21 and `collected in. the Working tank 2% from which .a .portion is delivered by pump Z9 through line 3S, controlled by valve Sii', to the arrester i9, as a quenching medium to terminate .the .decomposition reaction or" the vapors leaving the conversion .col Another portion of this liquid may be .returned to an intermediate point .in .the iractionating t Wer by pnnip 3i through line .32, as a control .medium for the inici-tower 4teImDeratur-.e if desired.

The overhead from the fractionating tower Ypasses .through .line .and cooler into a gasliduid separator .35, a portion of the condensate being returned Vto the top plate or the tower through line .S-'i' for temperature control. The remainder of thecondensate is Withdrawn from .the bottom of the separator through line 35, ccn- .trolled b y -valve .355, and sent to the motor fuel treating and finishing system, While the gas leaving the top or" the separator .through line ist, controiled by valve ai, goes to .the gas separating isystem.

I-claim:

1. A processior the conversion of higher-boiling hydrocarbons into lower boiling hydrocarbons, .the .steps which comprise: heating at non-- converting temperatures mixed high and los7 boilingv hydrocarbons to Vaporizing temperatures, thereafter separating the vaporized from the unvaporized hydrocarbons, passing the vaporized hydrocarbons through an elongated externally heated conversion zone or" restricted cross section, discharging the unvaporized hydrocarbons Without returning the same directly to said conversion zone; heating the vaporized hydrocarbons upon their introduction into said Zone and their vpassage therethrough to a desired conversion temperature, and maintaining such conversion temperature of said vaporized hydrocarbons substantially constant during the remaining passage thereof through said Yconversion sone by introducing into said hydrocarbons, after the latter have attained said conversion temperature, regulated Quantities of a duid coolant selected from the .group consisting of gaseous hydrocarbons, light liquid hydrocarbons, steam, and Water.

2. A method for converting hydrocarbons in the vapor phase as deined in claim 1, and Where- Y in vthe fluid .coolant employed in regulating temperature oi the vaporous hydrocarbons nnvdergoing conversion consists of water.

3. A method for converting hydrocarbons in the vvapor phase as donned in ciairn l, and wherein the fluid coolant employed in regulating the .temperature O :i the vaporous hydrocarbons undergoing Aconversdon .consists of steam.

e. A .method .or .converting hydrocarbons in the vapor phase as defined in claim l, and Where` .in the .duid coolant employed in regulating the .temperature Vof the :vaporous hydrocarbons un- Y verted hydrocarbons, and treating such vaporVV deleQ-ng conversion consists .of :.a low .boiling liquid hydrocarbon.

'5. A method for converting hydrocarbons in the vapor phase as defined in claim l, and wherein the lic-.id coolant employed in regulating the temperatnreof .the vaporous hydrocarbons undergoing conversion consists of a normally gaseous hydrocarbon.

A process for the conversion of higher .boiling hydrocarbons into lower boiling hydrocarbon-s, the steps which comprise: passing Ia stream of such higher boiling hydrocarbons while in a vaporized state through an elongated externally heated conversion zone of restricted cross section; heating the vaporized hydrocarbons immediately following their introduction into said zone toa desired conversion temperature in eX- cess of 10Goo F. and maintaining such conversion temperature of said hydrocarbons after it has been .attained substantially constant during the remaining passage of the hydrocarbons through said conversion sone by directly introducing into lsaid hydrocarbons in regni-ated quantities a iuid coolant selected from the groep consisting of gaseous hydrocarbons, light liquid hydrocarbons,

cam, and Water.

7. A process for the conversion or higher boiling hydrocarbons into lower boiling hydrocarbons, .the steps which comprise: passing a s reain oi such .higher boiling hydrocarbons While in a state through an elongated externally heated conversion Zone of restricted cross secti n, Aheating the vaporized hydrocarbons immediately following .their introduction into saidzone to a desired conversion temperature, maintaining said conversion temperature substantially uniform during .the remaining passage of the yathe duid coolant entering said conversion zone in a manner directly proportionate to the tempera-ture the vaporons hydrocarbons undergoing conyersion in said. Zone.

in the process set forth cla-iin 6, the step oi automatically introduci-ngthc huid coolant into the hydrocarbons in response the Variations in temperature of the hydrocarbon vaporLJ passing through vthe conversion zone.

9. A process for .the conversion oi higher boilhydrocarbonsinto lower boiling hydrocarbons, the steps which comprise: passing a stream of such higher boiling hydrocarbons while ina vaporiaed state through an elongated externally heated conversion zone of restricted cross section; heating the vapolfized hydrocarbons immediately following the' introduction into said Zone to a desired conversion temperature in excess of ifi-36 and maintaining such conversion telnerature of said hydrocarbons after it been .attained substantially constant during the remaining passage of the hydrocarbons through said conversion zone by directly introducing water as a coolant into said hydroca ons conversion zone in regulated quan es,V subsequently rapidity and substantiallyinstantaneously reducing the ternaeraturo of the vapors at'the kend or said convefsion zone to ,arrest abruptly ooonversion reactions, Yiractionating te .a vapor phase Vcracked `distillate from 'the con phase cracked distillate with snlphuric acid rre- 9 10 ning without excessive loss in volume or octane Number Name Date rating. 2,032,861 Watson Mar. 3, 1936 WILLIAM W. HOLLAND. 2,078,407 Ormont Apr. 27, 1937 2,240,160 Kaplan Apr. 29, 1941 References Cnted in the le of this patent 5 2,252,739 prickett et a1 Aug. 19, 1941 UNITED STATES PATENTS 2,431,485 Keeling Nov. 25, 1947 Number Name nate OTHER REFERENCES 1,698,811 Forward Jan, 15, 1929 Chemim Benning of getmleum, Kanchev- 1,811,194 Wagner June 23, 1931 10 sky et a1., pages 46, 47 and a'z, chem. catalog co.,

1,904,133 G0111 Apr. 18, 1933 New York (1933)

Claims (1)

1. A PROCESS FOR THE CONVERSION OF HIGHER-BOILING HYDROCARBONS INTO LOWER BOILING HYDROCARBONS, THE STEPS WHICH COMPRISE: HEATING AT NONCONVERTING TEMPERATURES MIXED HIGH AND LOW BOILING HYDROCARBONS TO VAPORIZING TEMPERATURES, THEREAFTER SEPARATING THE VAPORIZED FROM THE UNVAPORIZED HYDROCARBONS, PASSING THE VAPORIZED HYDROCARBONS THROUGH AN ELONGATED EXTERNALLY HEATED CONVERSION ZONE OF RESTRICTED CROSS SECTION, DISCHARGING THE UNVAPORIZED HYDROCARBONS WITHOUT RETURNING THE SAME DIRECTLY TO SAID CONVERSION ZONE; HEATING THE VAPORIZED HYDROCARBONS UPON THEIR INTRODUCTION INTO SAID ZONE AND THEIR PASSAGE THERETHROUGH TO A DESIRED CONVERSION TEMPERATURE, AND MAINTAINING SUCH CONVERSION TEMPERATURE OF SAID VAPORIZED HYDROCARBONS SUBSTANTIALLY CONSTANT DURING THE REMAINING PASSAGE THEREOF THROUGH SAID CONVERSION ZONE BY INTRODUCING INTO SAID HYDROCARBONS, AFTER THE LATTER HAVE ATTAINED SAID CONVERSION TEMPERATURE, REGULATED QUANTITIES OF A FLUID COOLANT SELECTED FROM THE GROUP CONSISTING OF GASEOUS HYDROCARBONS, LIGHT LIQUID HYDROCARBONS, STEAM, AND WATER.

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